The present invention relates generaly to fuel assemblies for nuclear reactors and, more particularly, is directed to an improved grid structure for selectively mounting a fuel rod lateral support means or a coolant flow deflecting means on a predetermined number of the fuel rods within a fuel assembly.
In most nuclear reactors the core portion is comprised of a large number of elongated fuel elements or rods grouped in and supported by frameworks referred to as fuel assemblies. The fuel assemblies are generally elongated and receive support and alignment from upper and lower transversely extending core support plates. These upper and lower core support plates are directly or indirectly attached to a support barrel which surrounds the entire core and extends between the ends thereof. In the most common configuration, the axis of the core support barrel extends vertically and the various fuel assemblies are also arranged vertically resting on the lower support plate. Generally, in most reactors, a fluid coolant such as water, is directed upwardly through apertures in the lower core support plate and along the various fuel assemblies to receive the thermal energy therefrom. Conventional designs of these fuel assemblies include a plurality of fuel rods and control rod guide thimbles held in an organized array by grids spaced along the fuel assembly length and attached to the control rod guide thimbles. Top and bottom nozzles on opposite ends thereof are secured to the control rod guide thimbles in thereby forming an integral fuel assembly. The respective top and bottom nozzles extend slightly above and below the ends of the fuel rods, capturing the rods therebetween.
Generally, in each fuel assembly, there are a number of grids axially spaced along the fuel assembly length and transversely extending across the assembly. Conventional designs of these grids include a plurality of interleaved straps of egg-crate configuration designed to form a plurality of cell openings, with each cell opening adapted to receive therethrough one of the fuel rods. A peripheral strap, being of the same height of the interleaved straps, encloses the interleaved straps to impart strength and rigidity to the grid. The purpose of these grids is two-fold. One purpose is for the lateral support or positioning of the fuel rods so as to prevent localized neutron flux peaking and thereby permit operation of the reactor closer to its design power limit. The other purpose is for the mounting of deflecting vanes to promote mixing of the upwardly flowing coolant among the fuel rods to average the enthalpy/temperature rise for maximizing the power output of the reactor core. Normally, for lateral support or positioning of the fuel rods, each grid cell opening includes an arrangement of spring fingers and dimple protrusions which provide a six-point contact of the fuel rod, such as seen in the positioning grids shown in U.S. Pat. Nos. 3,255,091 and 3,379,617. For deflecting the coolant flow, some or each of the cell openings of the grids are provided with cantilevered deflecting vanes such as shown in the mixing grids of U.S. Pat. No. 3,395,077. Still other grids, such as the ones shown in U.S. Pat. Nos. 3,379,619 and 4,061,536, include means for laterally supporting the fuel rods, as well as, means for deflecting the coolant flow. All of these prior art grids, extend completely across the fuel assembly and separately surround each of the fuel rods contained in the assembly. Furthermore, the construction of each of these grids is such that its outer peripheral strap is of a height equal to the height of its inner straps.
The power output of a nuclear reactor is limited by the rate at which heat can be removed from the reactor core, and the rate of heat transfer determines the temperatures developed in a reactor core. Therefore, the maximum reactor operating power is limited by some enthalpy and/or temperature value in the reactor core. The variation of the neutron flux in the reactor core causes the fuel assemblies in the core to operate at different power levels, and this variation occurs even among the fuel rods within a single fuel assembly. The reactivity and, in turn, the power output of a nuclear reactor is limited by the amount of structural material in the reactor core, as the structural material parasitically absorbs neutrons which could otherwise be used in the fission process. Further, a reduction of the structural material in a fuel assembly reduces the pressure drop and thereby increases the power output. Still further, it is well known, that the burn-up rate for the different fuel rods contained in a given fuel assembly varies. And still further, the output of a given fuel assembly can be enhanced by the use of different fissionable materials, as well as, by the amount of fissionable material, such as, through the use of different diameter sized fuel rods. With these considerations in mind, designers are constantly striving to improve upon the power output of the various fuel assemblies which make up the core to increase the total power output of the reactor, while at the same time, striving to improve on the construction of the assembles so as to facilitate the assembling of a fuel assembly and to reduce the repair and maintenance costs associated with operating the reactor.